Boot covers provides an extra layer of protection
When the weather gets really cold and tough, a boot cover gives you that extra layer of protection. The bigger brands like Salomon and Madshus have their own boot covers that fits perfectly on their models, but there's no trouble in choosing a boot cover for your boots from another manufacturer than your boots.
Besides keeping you feet warm, a set of boot covers will also give your feet an extra layer of protection in slushy conditions and provide you with a pair of waterproof boots. Almost every pair of modern ski boots are close to being waterproof, but in extreme conditions you can never be too careful.
A boot cover also gives you the opportunity to placing chemical warmers in between your boots and the covers. This will give you even more warmth and protection in extreme conditions.